1. Field of the Invention
This invention relates to a process for separating at least Fe(III) from an aqueous metallic salt solution containing at least Fe(III) using a certain organic extraction solvent, and a process for stripping Fe(III) from an organic extraction solvent loaded with Fe(III) using a mineral acid.
2. Description of the Prior Art
Aqueous solutions containing ferric ions are obtained from, for example, hydrometallurgy of ores, metal concentrates, etc.; using a mineral acid, resulting in acidic aqueous solutions containing ferric ions and other useful metal ions. Such aqueous solutions are also obtainable from the purification of metallic salts, industrial wastes, etc.
The recovery of useful metals from such acidic aqueous solutions is carried out by the addition of an alkali to the acidic aqueous solutions to thereby precipitate ferric ions as hydroxides. However, this conventional precipitation method requires the separation of hydroxides, which are difficult to filter, from the liquid phase. Also according to this conventional precipitation method, losses of the target metals due to the coprecipitation thereof with the hydroxides are not small. To eliminate these drawbacks, a solvent extraction method has been proposed wherein Fe(III) is extracted from an aqueous solution of metallic salts using an extraction solvent obtained by dissolving di-2-ethylhexyl phosphoric acid (D2EHPA) in kerosene. According to this method, however, in order to strip Fe(III) from the Fe(III) loaded extraction solvent, it is essential to bring the said extraction solvent in contact with 6N-hydrochloric acid. the use of 6N (or more)-sulfuric acid instead of 6N-hydrochloric acid results in insufficient stripping of Fe(III).
Similar solvent extraction methods have been also applied to the recovery or the purification of other metals from the remaining aqueous solution using an extraction solvent, D2EHPA. However, since iron of several milligrams per liter is still left in the remaining aqueous solution, which is also contaminated by iron from the corrosion of the extraction equipment, iron is increasingly accumulated in the extraction solvent so that it is unavoidably admixed into the other metals to be extracted when the extraction solvent is recycled to extraction processes.
Thus, it is desirable to extract Fe(III) prior to other metals and use an extractant with which a mineral acid, especially sulfuric acid, can be employed in a low concentration, so as to make stripping of Fe(III) more efficient.